This is a continuation of PCT/NL91/00161 filed on Aug. 30, 1991.
1. Field of the Invention
The invention relates to known loudspeakers of the type comprising a magnet unit which generates a magnetic field and an element suspended for resilient movement round a stable rest position, such as a diaphragm, a cone, a dome or other element suitable for radiating sound with selected properties. The known loudspeaker further comprises an electrical conductor, for instance a speech coil, connected to the movable element and having current feed terminals, wherein the conductor is placed in the magnetic field such that when current is passed through the conductor a force is exerted on the movable element which imparts to the element a deviation from its rest position.
2. Description of the Related Art
Reference is made for instance to the published Netherlands patent application 85.01166. Described herein is a ribbon speaker comprising a diaphragm which is clamped in a frame and in the central region of which is situated a zone containing the conductor arranged in a selected pattern.
A loudspeaker of this type is capable of radiating a certain minimal frequency sound that satisfies high quality requirements in respect of uniformity and little distortion. A drawback to such a loudspeaker however is that it suffers from power compression. This is the undesired phenomenon occurring when, due to current passing through the conductor, the conductor is subject to a considerable temperature increase which is attended by an increase in its resistance. When the voltage on the current feed terminals remains the same, the generated acoustic power decreases with such an increased resistance. Compression thus occurs.
A further drawback of the known loudspeaker is that it is capable to only a very limited extent, through natural convection, conduction and radiation, of draining the heat from the conductor. This phenomenon in combination with the relatively small output has the consequence that the loudspeaker can only generate a limited acoustic power. Loading with an impedance converter such as a horn can considerably enlarge the generated sound pressure in a limited space angle, but this does not essentially resolve the described limitation.
While the mentioned drawbacks and limitations are described with reference to a loudspeaker of the isophasic type, they also occur in other electrodynamic loudspeakers, for example a loudspeaker of the type with a magnet unit with cylindrical air gap in which a speech coil is axially movable, which speech coil drives a diaphragm or cone. In multi-channel systems such a loudspeaker is normally used for the low tones.